Thermionic amplifier



July 5, 193s.

E. GREEN THERMIONIC AMPLIFIER Filed Oct. 29, 1932 2 Sheets-Sheet l EMME BY l r M014,

ATTORNEY July 5, 1938. v E GREEN 2,122,772

THERMIONIC AMPLIFIER Y Filed Oct. 29, 1952 2 Sheets-Sheet 2 c; z l

v L +f :i l, :L l/ y E; i is 2 INVENTOR ERNEST G BY /f l ATTORNEY Patented July 5, 1938 UNITED STATES THERMONIC' AMPLIFIER Ernest Green, Chelmsford, England, assigner to Radio Corporation of America, a, corporation of Delaware Application October 29, 1932, Serial No. 640,280 In Great Britain October 29, 1931 3'v Claims.

This invention relates to thermionic amplifiers and more particularly to thermionic amplifiers of what may be termed the multi-tube or multi-valve per stage type i. e. amplifiers wherein a plurality of tubes are provided in a single stage of ampliflcation.

In thermionic ampliiiers embodying a plurality of tubes in a single stage of amplification, and as at present in common use, difficulty is often eX- perienced in practice, owing to the fact that the individual tubes will not share the alternating current load equally unless their electrical characteristics are identical. For example, where a single stage of ampliiication comprises a plurality of triodes in parallel, if theseparate tubes are of slightly different internal characteristics they will not ordinarily share the alternating current load equally with one another. For this reason it is usual to require that tubes which are intended to work together in a single multitube stage, shall be matched, but this is always a more or less expensive requirement, and where large tubes are in question the additional expenditure may be prohibitive.

Similar requirements as regards matching for alternating current load apply not only to parallel tube circuits but also to so-called pushpull circuits as ordinarily employed.

It is, of course, also desirable in most cases to match as regards the direct current components of anode or plate current, but this presents no difliculties, and may be accomplished quite simply by the usual method of providing where necessary, different values of grid bias for the different tubes e. g. by providing grid bias batteries of diiferent Values for the different tubes or, where automatic grid bias is in question, by arranging the tube grid circuits to receive different values of potential from a grid biasing resistance or its equivalent.

The present invention has for its object to provide an improved arrangement whereby the necessity for accurately matching thermionic tubes in a multi-tube amplifier stage is avoided.

The invention is applicable alike to parallel tube circuits and push-pull tube circuits and provides a degree of balance of such accuracy as to render the said invention applicable to circuits of large power. This applicability to circuits of large power is very important for it will be appreciated that, for example, in the case of tubes in parallel, it is possible if any substantial disparity between tubes exists, for the individual alternating current loads of the tubes to Vary so much so that in some circumstances the load on some of the tubes may even be negative, i. e. some of the tubes may drive the others.

According to this invention, the input electrodes of a plurality of tubes arranged in a common amplifying stage are so connected to a preceding stage of amplification or other source of Voltage to be amplified that the input alternating current voltage applied to different tubes is different and is in each case suited or adjusted to the characteristics of the tube in question, so that the said tube will take its correct proportion of the total alternating current load.

The invention is illustrated in and explained in connection with the accompanying drawings which show diagrammatically various arrangements in accordance therewith.

Fig. l is a circuit diagram of a parallel connected resistance coupled ampliiier;

Fig. 2 is a diagram of a theoretical circuit of the input voltages;

Fig. 3 is a circuit diagram which is a modication of Fig. l;

Fig. 4 is a circuit diagram of a transformer coupled ampliiier;

Fig. 5 is a circuit diagram of a push-pull amplilier;

Fig. 5 is a circuit diagram of another modification of the circuit arrangement shown in Fig. 3.

Referring now to Figure l which shows one way of carrying out the invention, a parallel anode connected resistance-capactiy coupled thermionic amplier comprises an input impedance consisting of the usual resistance M one end of which is connected at GB- to the negative terminal of a suitable grid bias source (not shown) and the other end of which is connected through the usual coupling capacity C1 to a preceding stage or other source of voltage to be amplied. The parallel stage now described consists of three triodes V1 V2 V3 in parallel (though of course there may be any other number of tubes) the anodes of the triodes being connected together and through an anode resistance R to the positive terminal of a source of anode potential (not shown) the junction point of the anode resistance R and the anodes being also connected through the customary coupling condenser C2 to the next stage.

The three grids of the tubes are eifectively connected each to a different tapping point T1 T2 or T3 on the input resistance M the leads to the grids of the tubes Vi, V2, containing each a supplementary bias battery b1 or b2 to maintain equal direct current anode feeds to the three tubes.

The cathodes are connected together and to thei negative terminal of the anode potential source in the usual way.

The above described circuit is, from the point of view of the present invention, the electrical equivalent of the theoretical circuit shown in Figure 2 which theoretical circuit consists of a plurality of parallel paths each path consisting of an alternator, and a resistance in series, the three parallel paths being in parallel across a common load resistance R. The voltage of the alternator in each branch of the equivalent circuit is, as indicated in Figure 2, the product of the alternating input voltage eg1 or egz or egg to the appropriate tube multiplied by the amplification factor 1u, ,uz or la of that tube.

The resistances in series with the equivalent alternators are the internal resistances of the tubes in question and are represented by R1, R2, R3. The common load resistance is represented by R.

In carrying out the present invention the input tapping points are so chosen that the terminal voltages across all the alternator-resistance branches in the equivalent circuit are equal when the load is shared equally. This condition is obviously satisfied when the proper bias battery values are chosen and the direct current through the tube equalized a condition for obtaining equal alternating current through the tubes will then be where etc.

or expressed in general terms for the case Where there are n tubes The above condition is the condition for accurate adjustment. Clearly however, when the value of R is very much greater than a tube resistance divided by the number of tubes suicient accuracy will be obtained by making etC.

Y C2 or C3 connected on the grid side in series with a resistance Rc1-Roz or Rga the other end of which is connected to a point on a common grid bias battery B. The modulator stage consists of a plurality of tubes V1 V2 V3 having their anodes connected in parallel, each grid is connected to a tapping point T1 T2 or T3 upon the appropriate resistance there being, of course, as many parallel input condenser circuits as there are tubes. The anodes of the tubes are connected together and through a choke L to the positive terminal of the anode potential source (not shown) the high frequency stage HFS to be modulated being connected as in the usual way between the common anode point and the common cathode point of the modulator stage.

The invention is also applicable to transformer coupled amplifiers and in one such circuit arrangement shown in Figure 4, the grid of each tube V1", V2, Vs of a parallel connected stage is connected through a coupling condenser C1', C2', C3 to a different point on the tapped secondary TS of an input transformer. The grid side of each coupling condenser is connected througha separate resistance R1,R2, R3" to a suitable point on a common bias battery B' vand the output from the stage is taken off as in the usual way through an output transformer OT whose primary is connected between the common anode point and the high tension source (not shown).

In a still further circuit modification shown in Figure 5' in which the invention is applied to a four tube amplifier consisting of two tubes V1, Vz', and V3, V4 in parallel and two (V1 Va" and V2' V4") in push pull each grid is coupled through its own coupling condenser Cl'",

Cz'", 03 or 04" to a different tapping upon the secondary TS of an input transformer the push tubes being connected to different tapping points upon one side of the electrical centre EC of the transformer secondary and the two pull tubes being connected to points upon the other side of the electrical centre. Each grid is also directly connected to its o-Wn grid resistance R91 Rgz Rgs or R94 the other end of which is connected to a suitable point upon a common bias battery B the cathodes of the four tubes being connected together and to the electrical centre of the transformer secondary. The output circuit of the stage consists of a centre tapped output transformer OT arranged as in the usual way.

Figure 6 of the accompanying drawings shows a still further circuit arrangement in accordance with this invention, this circuit being a modification of the arrangement shown in Figure 3. In Figure 6, V1 V2 V3 are three tubes connected in parallel, each tube having a separate grid resistance R91 R92 or Roa the ends of these resistances remote from the respective grids being connected to tapping points upon a common bias battery B.. Input to the grids of the tubes V1' V2 V3 is taken through coupling condensers C1 C2 C3' from tapping points T1 T2 T3 upon an anode resistance AR connected in the plate circuit of a preceding tube PV.

The invention is obviously applicable to stages consisting of any desired member of tubes in parallel or in push-pull or any combination of parallel and push-pull.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:-

1. An amplifying system comprising a plurality of tubes, each having an anode, a cathode and a grid, means connecting the anodesof the tubes directly in parallel, means connecting together the cathodes of said tubes, an alternating current output circuit connected between the parallelly connected anodes and said cathodes, an alternating current input circuit, means connecting the cathodes of said tubes to said input circuit, means connecting the grids of said tubes to points of different alternating current potential in said input circuit, and means for subjecting each grid to a different direct current potential with respect to its associated cathode.

2. An amplifying system as described in claim 1 characterized by the fact that the grids are connected to the input circuit by way of a plurality of direct current blocking condensers.

3. An amplifying system as described in claim 1 characterized by the fact that the input circuit comprises a transformer, and also by the fact that the grids are connected to the secondary of said transformer by way of a plurality of direct current blocking condensers. 

